Partitioning of Highly Siderophile Elements at Elevated Pressure and Temperature: Constraints on Core Formation and Accretion of the Earth

Arizona State University, Scottsdale AZ

Investigators

Abstract

Sharp and Hervig EAR-0087584 It is proposed to quantify the partitioning of the highly siderophile elements Au, Pt, Ir, and Os between silicate liquid and iron-sulfide liquid in chondritic material quenched from experiments at high pressures and temperatures. Such partitioning data will allow us to better constrain models for the accretion and differentiation of the early Earth. The conditions of the experiments (P = 1 to 25 GPa, T up to 2500 C) include values appropriate to a magma ocean presumed to have been generated during the early stages of formation of the Earth. Because the Earth accreted from volatile-depleted chondritic material, we will use a grade 5 or 6 H chondrite as the starting material. The range of conditions proposed will allow us to investigate the effect of pressure and temperature on partitioning for a bulk composition similar to the accreting Earth. Although much high-pressure work has been done on the moderately siderophile elements, little work has been done to measure the partitioning behavior of highly siderophile elements (HSEs) at high pressure. If our results show that the HSEs are not highly fractionated into the iron-sulfide melt, then the present-day abundance of siderophiles can be explained by a homogenous accretion model with metal-silicate equilibration in a deep magma ocean. If our data show strong partitioning of HSEs into the metal-sulfide phase then the mantle abundances of HSEs must be explained by a lack of equilibrium between core and mantle and heterogeneous accretion. In order to estimate the contributions of either homogeneous or heterogeneous accretion models, we must know the partitioning behavior of siderophile elements under conditions appropriate for accretion and core formation from chondritic material. In addition to measuring partition coefficients, we will use high-resolution electron microscopy to characterize the run products to determine if the HSEs are dissolved in the silicate or occur as micro nuggets.